Added canonical test cases for invalid packet input (#29)

[vasilybib1]

Summary

• Add canonical test vectors for invalid packet inputs (trailing bytes, invalid enum values, invalid array sizes, invalid fixed values, invalid lengths)
• Update all backend test harnesses (Rust, C++, Java, Python) to exercise invalid test vectors
• Add missing validation checks to the C++ and Java backend decoders so they reject invalid inputs

Test Cases

15 invalid test vectors (LE), covering 5 error types across 6 packets:

InvalidLengthError (3 cases)

• Packet_Scalar_Field with empty input - packet expects 8 bytes, gets 0
• Packet_Scalar_Field with 000000 - packet expects 8 bytes, gets 3
• Packet_Array_Field_ByteElement_ConstantSize with 0102 - expects 4-byte fixed array, gets 2

TrailingBytes (2 cases)

• Packet_Scalar_Field with 000000000000000000 - 9 bytes for an 8-byte packet (1 extra)
• Packet_Array_Field_ByteElement_ConstantSize with 0102030405 - 5 bytes for a 4-byte fixed array (1 extra)

InvalidEnumValueError (5 cases)

• Packet_Enum_Field with 0x00 - not a valid tag in the closed enum
• Packet_Enum_Field with 0x05 - falls between valid tags
• Packet_Enum_Field with 0x7e - above all valid tags
• Enum_Incomplete_Truncated_Closed with 0x02 and 0x07 - invalid values for a closed enum with range tags

InvalidFixedValue (4 cases)

• Packet_FixedScalar_Field with 0x00 and 0x06 - wrong value for a field fixed to a specific scalar
• Packet_FixedEnum_Field with 0x02 and 0x04 - wrong value for a field fixed to a specific enum tag

InvalidArraySize (1 case)

• Packet_Array_Field_ScalarElement_VariableSize with 03010203 - size field says 3 bytes but element width is 2, so 3 % 2 != 0

The BE test vectors mirror these except Enum_Incomplete_Truncated_Closed (LE only), giving 12 BE cases.

Closes #29

[hchataing]

Thanks for the patch :)
Good to merge on my side.
Waiting for @jameseidson to validate the changes to the Java backend.

[jameseidson]

LGTM, thanks for the patch!